1. Field of the Invention
The present invention relates to methods for detecting, locating and measuring leaks in test objects, and more specifically, to a system using a differential absorption technique for detecting, locating and measuring leaks.
2. Description of the Prior Art
There is clearly a need for a more rapid, thorough and reliable leak detection approach for sealed systems, in general, and reusable rocket engines, in particular. The current approach used on the Space Shuttle Main Engine (SSME) involves a soap solution and is quite laborious, as well as incomplete, because only pre-selected points on the engine are inspected. An extension of this approach to all the hundreds of joints, welds, and brazes on the engine would be prohibitively time-consuming and is not practical. An alternative approach has been utilized which consists of surrounding the entire engine with an airtight bag and testing for helium leaks with a mass spectrometer. While more accurate and reliable for detecting the presence of leaks, this method is again quite laborious and, furthermore, gives no indication whatsoever of where the leaks are occurring. If leaks are detected, bagging must still be followed by time-consuming soap solution testing to identify the leak sources. The entire leak inspection process on the SSME consumes several workshifts and often must be repeated one or more times to reliably identify leaks and determine their sources. In contrast, turnaround times desired for future reusable space vehicles are on the order of one day or less, implying that total vehicle inspection times must be on the order of hours or less.
The application of optical imaging for visualizing leaks has been demonstrated by the present assignee, Rockwell International Corporation, using a holographic technique. U.S. Pat. No. 4,612,797 issued to Sarkis Barkhoudarian entitled, "Leak Locating and Mapping System and Method". In this technique, holographic records are made of a system under test before and after pressurization with a gas, and the records are superimposed, revealing interference fringe distortions where leaking gas appears.
Other optical techniques studied and tested by Rockwell International Corporation for the SSME program have included schlieren and speckle interferometry. In each technique, the engine under inspection is pressurized with an inert gas which then escapes from locations such as faulty joints or welds. Each technique is configured to provide an image of an engine section, captured by a video camera, in which the leaks appear as visible, localized disturbances. The techniques are based on different physical principles: In the case of schlieren, the disturbances are induced by the bending of collimated light by leaking gas; in the case of speckle interferometry, the disturbances are induced by minute shifts in the optical path length of laser light due to the leaking gas.
Each of the above techniques are very sensitive to vibration, air currents and small changes in optical alignment. Furthermore, they are limited in their sensitivity to small leaks and generally require complex imaging processes.